Semiconductor materials with narrow bandgaps are often sought to form the base layers of Gallium Arsenide (GaAs) heterojunction bipolar transistors (HBTs) in order to produce lower base resistances, turn-on voltages (Vbe,on), and offset voltages (Vce when Ice˜=0). In theory, a reduction in the offset voltage would also reduce the knee voltage and should provide Power Added Efficiency (PAE) improvements in a power amplifier application. However, the choice of available base materials is generally limited by the constraints of matching the lattice constant of the underlying GaAs substrate.
Poor lattice matching between layers in the semiconductor structure often results in lattice stresses that degrade performance. For example, Ga0.95In0.05N0.003As0.997 has been used as base material to reduce the turn-on voltage and offset voltage of an HBT while maintaining a usable current gain β of the HBT. Both Gallium Arsenide Antimonide (GaAsSb) and Gallium Indium Arsenide (GaInAs) have also been used as base materials in Indium Gallium Phosphide (InGaP) HBTs. These base materials did lower turn-on voltages, but resulted in lower current gains β due to higher base resistances.
Accordingly, semiconductor materials with narrow bandgaps less than GaAs are needed to provide lower base resistances, turn-on voltages, and/or offset voltages while providing adequate lattice matching with the underlying GaAs substrate.